FIG. 10 (a) is a plan view schematically illustrating a prior art light modulator module for explaining wiring to a semiconductor light modulator. In the figure, reference numeral 25 designates an integrated light modulator and laser chip comprising a semiconductor laser 2 and a semiconductor light modulator 1 that is optically coupled with the laser 2. Although it is not shown in the figure, the semiconductor light modulator 1 is optically coupled with a lens. The integrated light modulator and laser chip 25 is disposed on a submount 9. A bonding pad 21 for the light modulator 1 and a bonding pad 22 for the laser 2 are disposed on a side of the submount 9. The submount 9 is disposed on a carrier 20 comprising a conductor.
On the carrier 20, an alumina dielectric strip 23 having a prescribed thickness is disposed parallel to with the submount 9, and a strip conductor 24 having a prescribed width is disposed in the center of the surface of the alumina dielectric 23. The carrier 20, the alumina dielectric 23, and the strip conductor 24 form a strip line 3 that is a kind of plane-parallel transmission line. For example, the strip line 3 comprising a 250 .mu.m wide strip conductor 24 and a 250 .mu.m thick alumina dielectric 23 and has a characteristic impedance Z.sub.0 of 50 .OMEGA.. A terminating resistor 5 comprising a thin film resistor is fabricated on a terminal portion of the alumina dielectric 23. An end of the terminating resistor 5 is connected to a first terminal of the strip conductor 24 while the other end thereof is connected to a grounded through-hole 6. The resistance of the terminating resistor 5 is 50 .OMEGA., that is, equivalent to the characteristic impedance Z.sub.0 of the strip line 3 to make the strip line 3 a matching line. A second terminal, opposite the first terminal, of the strip conductor 24 is connected to a signal supply 8. A signal input terminal of the semiconductor light modulator 1 is connected to the first terminal of the strip conductor 24 via the bonding pad 21 with a wire 4. Preferably, a gold wire of diameter 25 .mu.m is used as the wire 4. A feeding wire 7 for supplying an electrical current to the semiconductor laser 2 is connected to the semiconductor laser 2 via the bonding pad 22.
FIG. 10(b) is an equivalent circuit diagram of the light modulator module shown in FIG. 10(a), through which a high-frequency electrical signal input to the circuit from the signal supply 8 is transmitted to the semiconductor light modulator 1. In this equivalent circuit diagram, reference character L.sub.1 denotes an inductance of the wire 4, C denotes a capacitance of the semiconductor light modulator 1, R denotes a resistance of the terminating resistor 5, and Z.sub.0 denotes a characteristic impedance of the strip line 3. The strip line 3 having the characteristic impedance Z.sub.0 is connected to the signal supply 8. The resistor R and a series circuit, comprising the inductance L.sub.1 and the capacitance C, are connected in parallel, and a node of the resistor R and the series circuit is connected to the strip line 3. The inductance L.sub.1 of the wire 4 is proportional to the length of the wire 4.
A description is given of the operation of the prior art light modulator module. Laser light emitted from the semiconductor laser 2 is input to the semiconductor light modulator 1. Since high-frequency electrical signals are applied through the strip line 3 and the wire 4 to the signal input terminal of the semiconductor light modulator 1, the laser light is modulated in response to the high-frequency signals and emitted to a lens. The high-frequency signals have a frequency band of 2.about.10 GHz.
When the semiconductor light modulator 1 modulates laser light with high-frequency electrical signals, if the impedance of the circuit viewed from the signal supply 8 is not equivalent to the impedance of the signal supply 8 viewed from the circuit, some of the high-frequency signals input to the circuit from the signal supply 8 do not reach the light modulator 1 and return to the signal supply 8. This phenomenon is called a high-frequency reflection or a return loss S11 and the return loss S11 is defined as EQU S11=10 log (reflection power/input power)
As can be seen from FIG. 10(b), this return loss S11 is significantly influenced by the wiring to the semiconductor light modulator 1 because the impedance of the circuit viewed from the signal supply 8 depends on the inductance L.sub.1 of the wire 4. When the return loss S11 is large, the waveform of the high-frequency electrical signal is distorted. So, the return loss S11 is desired to be as small as possible.
However, in the prior art light modulator module, for example, the return loss S11 at 2.5 GHz is as high as -5 dB, and it is necessary to reduce the return loss to about -10 dB.